Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely proposed for large-scale energy storage applications. Various battery designs, including flow batteries, have been considered for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing sides of a membrane or separator in an electrochemical cell containing negative and positive electrodes. The flow battery is charged or discharged through electrochemical reactions of the active materials that occur inside the two half-cells. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or variants thereof will synonymously refer to materials that undergo a change in oxidation state during operation of a flow battery or like electrochemical energy storage system (i.e., during charging or discharging). Although flow batteries hold significant promise for large-scale energy storage applications, they have often been plagued by sub-optimal energy storage performance (e.g., round trip energy efficiency) and limited cycle life, among other factors. Despite significant investigational efforts, no commercially viable flow battery technologies have yet been developed.
Metal-based active materials can often be desirable for use in flow batteries and other electrochemical energy storage systems. Although non-ligated metal ions (e.g., dissolved salts of a redox-active metal) can be used as an active material, it can often be more desirable to utilize coordination compounds for this purpose. As used herein, the terms “coordination complex, “coordination compound,” and “metal-ligand complex” will synonymously refer to a compound having at least one covalent bond formed between a metal center and a donor ligand. The metal center can cycle between an oxidized form and a reduced form in an electrolyte solution, where the oxidized and reduced forms represent states of full charge or full discharge depending upon the particular half-cell in which the coordination compound is present. Because the oxidation-reduction cycle of many coordination compounds involves the transfer of only one electron at the metal center (i.e., a change in metal oxidation state of +1 or −1), the amount of charge that can be stored is frequently limiting, particularly for large-scale applications. That is, the ratio of transferred electrons per unit of coordination compound is low on a molar basis, typically a 1:1 ratio. Since solubility in an electrolyte solution is frequently a limiting parameter for coordination compounds and other metal-based active materials, there can be minimal opportunities to increase the energy density of a given flow battery configuration by altering the active material's concentration. Even when coordination compounds are used near their saturation concentration in an electrolyte solution, the energy density can still remain undesirably poor due to the low ratio of electrons transferred per unit of active material on a molar basis. Further, operating near an active material's saturation concentration can be precarious due to precipitation concerns. Not only can precipitation decrease the energy density of flow batteries by lowering the amount of available active material, but precipitation can also be problematic due to potential occlusion of circulation pathways and damage to other cell components.
In view of the foregoing, active materials for promoting enhanced energy density values in flow batteries would be highly desirable in the art. The present disclosure satisfies the foregoing need and provides related advantages as well.